Much of the electrical power used in homes and businesses throughout the world is produced in power plants that burn a fossil fuel (i.e. coal, oil, or gas) in a boiler. The resulting hot exhaust gas (also sometimes termed “flue gas”) turns a gas turbine or boils water to produce steam, which turns a steam turbine, and the turbine cooperates with a generator to produce electrical power. The flue gas stream is subsequently passed through an air preheater, such as a rotating wheel heat exchanger that transfers heat from the flue gas to an incoming air stream, which thereafter flows to the combustor. The partially cooled flue gas is directed from the air preheater to the exhaust stack.
The flue gas contains contaminants such as nitrogen oxide (NOx) and carbon monoxide (CO) and particulates of soot when, for example, coal is used as the primary fuel source. The discharge of all of these contaminants into the atmosphere is subject to federal and local regulations, which greatly restrict the levels of these flue gas components.
To meet the reduced levels of NOx emissions from power stations required by environmental regulations, many fossil fuel-fired electric generating units are being equipped with selective catalytic reduction (SCR) catalysts. In SCR, the most common method used is to inject ammonia or urea based reagents in the presence of a vanadium oxide catalyst where the ammonia reacts to reduce the oxides of nitrogen. The SCR system typically operates at flue gas temperatures ranging between 300° C. and 450° C. U.S. Pat. No. 5,104,629 illustrates one known type of SCR installation.
In a typical power generation application, an oxidation catalyst is disposed upstream of the SCR catalyst for oxidation of the CO in the gas stream. The use of mixtures of NO+NO2 produced upstream of the SCR catalyst in the oxidation catalyst allows other reactions to occur in addition to the standard SCR reaction:4 NH3+4 NO+O2→4 N2+6 H2O (standard SCR reaction)  (1)4 NH3+2 NO+2 NO2→4 N2+6 H2O (fast SCR reaction)  (2)4 NH3+3 NO2→3.5 N2+6 H2O (slow NO2-SCR reaction)  (3).
Catalyst performance in SCR systems is affected by one or more of operating temperature, catalyst composition, space velocity, and molar ratio of NH3 to inlet NOx fed to the SCR catalyst. Particularly, at lower temperatures, for example, below 300° C., conversion of NOx in the SCR catalyst is impacted by the NO2 fraction of NOx. This behavior can be attributed to the fast SCR reaction (2) with a reaction rate higher than the standard SCR reaction (1) at lower temperatures. The remaining NO or NO2 reacts with ammonia according to the standard or NO2-SCR reaction, yielding the reduction of NOx of these reactions at the actual conditions of temperature and space velocity. At higher NO2 fractions, the reduction of NOx in the SCR catalyst decreases due to the different reaction rates of standard and NO2-SCR. Beside this impact, high NO2 fractions in the exhaust require more NH3 to be introduced into the SCR process, which increases the possibility of NH3 slip.
Typically, the oxidation catalyst upstream of the SCR catalyst in a power generation application consists of Pt or Pt/Pd on an alumina support washcoated onto a substrate such as a honeycomb or other suitable substrate. If NO to NO2 conversion at the oxidation catalyst is not controlled and is too high, the performance of the downstream SCR catalyst will be impacted. The typical Pt or Pt/Pd on alumina support oxidation catalyst tends to age over time causing the NO to NO2 conversion to increase. It would be desirable to provide an oxidation catalyst that provides acceptable CO conversion and acceptable levels of NO to NO2 conversion over time and do not show aging. Moreover, it would be desirable to provide methods and systems that control the conversion of CO and NOx in power generation systems.